As is well known, a loudspeaking telephone comprises a transmission channel incorporating a high-gain amplifier connecting the microphone to a two-wire subscriber line, and a reception channel incorporating a high-gain amplifier connecting the subscriber line to the loudspeaker. A differential coupler is used to connect the transmission and reception channels to the subscriber line, and, in theory, this prevents all leakage between the transmission and reception channels. However, as the matching of the differential coupler is necessarily less than perfect, the transmission signals break through into the reception channel, and this results in an electrical echo signal being present in the reception channel.
In addition to the electrical coupling between the transmission and reception channels at the differential coupler which connects these two channels to the subscriber line, there is also acoustic coupling between the loudspeaker and the microphone. The sound waves emitted by the loudspeaker impinge on the microphone, whether by virtue of direct or indirect acoustic coupling, in other words whether the sound waves are reflected by obstacles in the surrounding area or not. An acoustic echo signal is produced at the microphone. These echo signals generate self-oscillations which produce howling noises (the Larsen effect) which make telephone conversation difficult if not impossible.
To overcome the Larsen effect, there have been various methods proposed for isolating the transmission and reception channels from one another during a conversation, in other words to make one of the channels inactive during the operation of the other. One of the best known of these methods consists in introducing a certain level of attenuation into the transmission channel or the reception channel, depending on whether the distant or local end subscriber is speaking. This method uses so-called "voice-operated gain adjusting devices" to operate the equipment in alternating mode, in other words in transmission or in reception.
The use of such devices has certain disadvantages, however. If the operating mode switchover function operates at high-speed, the conversation becomes unpleasant, speech being cut up into separate syllables and brief noises resulting in unnecessary switching operations. If the operating mode switching is too slow, however, there is a risk that the first one or more syllables of a reply will be lost, and it is not possible for the other party to break in, the reception channel of the speaking party being rendered totally inactive.
The aim of the present invention is to attenuate the echo signals in order to eliminate the Larsen effect whilst avoiding chopping of the speech signals and making it possible for the listening party to interrupt the speaking party, in other words permitting the two parties to communicate in a natural manner.